Generally a magnetic recording medium (hereinafter sometimes called "a magnetic tape") comprising a non-magnetic support having provided thereon a magnetic layer containing ferromagnetic metal oxide particles comprised of acicular crystals such as .gamma.-Fe.sub.2 O.sub.3, Co-containing magnetic iron oxide or CrO.sub.2 disspersed in a binder is used as a magnetic recording medium such as an audio tape, a video tape or a computer tape. Particularly with the recent demand for higher density recording, a magnetic tape using ferromagnetic metal fine particles instead of ferromagnetic metal oxide particles has been used.
Recording of information and reproducing information are accomplished by running moving the magnetic recording medium in a manner so that a surface of the magnetic layer contacts a magnetic head. Improvement of running durability of a magnetic layer is strongly desired, because recording and reproducing are done at an extremely high speed and quite often.
Polyurethane resins of the polyester type which are excellent in wear resistance are generally used as binder resins for forming a magnetic layer to obtain running durability. However, polyurethane resins of this type are easily hydrolyzed, and are easily deteriorated by corrosive gas, moisture, light and heat. When this kind of resin is stored and used for a long period of term, for example, when this resin is allowed to stand under hard conditions, such as in a car in summer, the magnetic layer tends to deteriorate, and not only running durability but also storage stability are insufficient. Particularly, there is a problem of durability such as running durability after a long storage.
Recently, polycarbonate polyurethane resins have been used as polyurethane resins instead of the polyester type polyurethane resins because they are improved regarding hydrolysis resistance. A magnetic recording medium using a polycarbonate polyurethane resin is disclosed in JP-A-58-60430, JP-A-59-198530 and U.S. Pat. No. 4,643,949. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, the polycarbonate polyurethane resin has a number of problems in that materials for preparing the resin are expensive and limitative, and the reaction conditions for preparing the resin are undesirably complicated. Thus, it is very difficult to obtain the resin having desired physical strength.
A magnetic recording medium wherein a polyether polyester polyurethane resin containing in part a polyether polyol which is hydrolyzed to a small extent has been used as a polyester type polyurethane resin having improved hydrolysis resistance, as disclosed in JP-A-61-104325. However, in this case, the above noted defect of the polyester polyurethane resin that it is easily hydrolyzed could not completely be avoided. Running durability at high temperatures and high humidities and durability after a long storage without deteriorating electromagnetic characteristics and running property were not satisfactory.